The present invention relates to an apparatus mounted on a vehicle and arranged to provide a signal indicative of the spacing between the vehicle and a metal article being less than a predetermined amount, the predetermined amount being small compared with the length of the vehicle.
A circuit is known which operates with a capacitive measurement value pick-up which must be adjusted by means of check capacitance. An adjustment of this kind is awkward and dependent on numerous factors.
U.S. Pat. No. 3,688,186 describes an equipment for the location of faults in metal pieces, which operates with a delayed regulating circuit. This circuit is not suitable as a parking aid for motor vehicles.